This invention relates to a system for nondestructive testing and evaluation of material flaw characteristics. Modern technology places increasing demands on nondestructive evaluation (NDE) capabilities. In large measure these capabilities depend upon methods for non-invasive inspection of the interior of critical structures. It is necessary to infer the presence or absence of structural defects by interpretation of the energy emitted by or, more usually, reflected from these defects. Many forms of energy sources exist for NDE applications, including ultrasonic, eddy current, and electromagnetic, to name the most commonly used. However, in the prior art, interpretation of the waveforms of the emitted or reflected energy has been unsophisticated.
For example, the almost universal practice in acoustic emission monitoring has been to monitor the cumulative number and rapidity of pulse-like acoustic emissions from structures as their loads are applied. Little attention has been directed to consideration of the waveform shapes of the acoustic pulses, which may convey vital information about the true integrity of structures. As a further example, ultrasonic pulse-echo testing has relied almost entirely on just one parameter of reflected ultrasonic waveforms: amplitude. This is somewhat analogous to a mother trying to judge if her baby is ill by how loudly the infant cries. Amplitude can be indicative in some cases, but very misleading in others. Recent theoretical and experimental evidence in NDE research and development shows clearly that the reflected waveform shape, or, more expressly, the waveform frequency content of an ultrasonic echo, is much more significant as an indicator of the presence of a defect than is the amplitude.
Use of information in the shapes of NDE waveforms requires test-instrument systems and procedures not found in the prior NDE art. Of particular importance in the new art is realization of a high degree of automaticity, because human inspectors are not generally able to interpret the greatly increased quantity of data involved in waveform shape analyses. The achievement of automatic interpretation of NDE waveforms is a major thrust of the present invention.
This automatic interpretation capability is not limited to NDE applications, although those are extensive. The techniques of the present invention can be used anywhere when given events (waveform sources) and the resulting waveforms are uniquely and repeatedly related. Thus, the interpretation of physiological waveforms falls within the purview of our invention, as do processes in seismology, remote sensing of intruders, diagnoses of the condition of motor vehicles, and numerous other arts and disciplines.
Accordingly, NDE applications are discussed in the present disclosure by way of illustration but not limitation.
Likewise, there are many NDE applications envisioned by the inventors hereof. For example, in the aviation industry, the detection and sizing of underfastener cracks are critical inspection problems in airframe and turbine disk testing. For such, the NDE instrument system of the present invention enhances inspections because it provides: (1) ability to detect smaller defects than before, (2) quantitative measurements of defect sizes and orientations, (3) increased speed of inspection, and (4) reduced reliance on skill of the inspector. Other pressing needs in aeronautics and in weapons fabrication include inspection of adhesively bonded structures to detect and classify bond defects such as disbonds, delaminations, and bond porosity, and to infer the strength of bonds from parameters of ultrasonic signals.
The area to be used as the primary basis for discussion in this disclosure is the inservice inspection (ISI) of piping, nozzles, and pressure vessels. This embraces NDE applications for pipelines, boilers of all kinds, and the facilities used in fossil-fueled and nuclear-power generating plants. Several very critical ISI problem areas exist in these applications, among them:
(1) inspection for cracks and faulty welds in gas and petroleum pipelines;
(2) inspection of steam tubing for cracking, pitting, high-cycle fatigue, and denting (Denting is the term used to refer to the outcome of a corrosion process occurring on the secondary side of a steam generator-corrosion products build up in the crevice region between the tube outer surface and the surfaces of supporting plates; as the thickness of these deposits grows the tube is compressed; cracks can occur in the tube and in the support plates.);
(3) inspection for cracks in welds and weld heat-affected zones in piping for nuclear power generating and other plants (The welds may involve bi- and tri-metallic junctions.);
(4) inspection for inner-diameter cracks produced by intergranular stress-assisted corrosion cracking and by other causes in piping for nuclear power generating plants and other plants;
(5) inspection for inner-radius cracks in the parent metal and in cladding (where used) of feedwater nozzles, control-rod drive nozzles, and other nozzles of nuclear power generating and other plants; and
(6) inspections of reactor and other pressure vessels.
The motivation for improvement of ISI technology in the electric power generating industry comes from economic factors, the need to minimize radiation exposure to inspection personnel, the need for increased efficiency and reliability of ISI, and the goal of obtaining licensing credit from regulatory authorities. This is discussed fully in Planning Support Document for the EPRI Non-destructive Evaluation (NDE) Program, G. J. Dau, Electric Power Research Institute, Palo Alto, California, Sept. 2, 1977, which states "Experience to date indicates that present practice does not produce results of sufficient accuracy and repeatability to permit reliable flaw identification and classification," and that ". . . a shortage of skilled operators (inspectors) is imminent." The needed improvement in ISI NDE capabilities for the electric power industry will minimize the inspection time (especially in hostile environments); increase on-line analysis capability; speed up regulatory decisions; provide fast, accurate, reproducible, and unambiguous results to limit the escalation of test and inspection requirements; and prevent downtime via early warnings of material degradations and simplifying the inspection procedures, as skilled personnel are in short supply.
Presently, materials are ultrasonically (UT) inspected almost entirely by manual means. The human inspector is often in a biologically hostile environment due to heat, radiation, cramped working quarters, etc., yet must be vigilant and skilled in interpreting readings produced by NDE equipment. He must constantly verify correct position and orientation of the UT transducer and that proper coupling is being made between transducer and the object under test to ensure that the appropriate signals are observed.
In present industrial ISI practice, the radio-frequency UT signal is full-wave rectified by the inspector's (portable) receiver and displayed on a cathode ray oscilloscope as a series of spikes. During preinspection calibration tests, the inspector is instructed to report "indications" that are a certain percentage of full-scale height. Thus, the rectified echo amplitude occurring from a certain depth in the material is the basis of present ultrasonic NDE inspection practice. Unfortunately, many frequently-occurring factors can contribute to false indications (false alarms) and to no indications (false dismissals). These factors include inadequate couplant, surface roughness, material anisotropy, weldments in the sound path, delaminations, cladding interfaces, benign reflectors (e.g., material grains, pipe counterbores, and other geometric factors), transducer variations, defect orientations other than normal to the sound beam, defect size variations, etc. Thus, defect detection via visual evaluation of a rectified pulse, recorded under harsh conditions and due to any of the above factors, is an extremely difficult task, even for experienced inspectors. Defect sizing is considerably more difficult.
UT NDE can be made less hazardous to humans and more reliable relative to defect detection and sizing by: (1) an automatic or semiautomatic transducer scanning apparatus to reduce inspector exposure and provide greatly enhanced positioning control; (2) a microcomputer-based signal processor to interpret the RF signals (without necessarily using amplitude as an input parameter) and to mask out the characteristics of different transducers; (3) apparatus for recording the unrectified (i.e., RF) echoes that enable an archieval copy to be made--the UT scan can be replayed later for further analyses, inspector training, and defect growthrate determinations.
These devices must be programmable for flexibility of use and must be integrated functionally within a single system that is physically compact and rugged. Microprocessors and microcomputers are used as interfaces in place of hardwired circuits to permit changes in recording, display, playback, and analysis functions, as conditions require. Adaptive Learning Networks (ALN's) are used to detect defects and discriminate between them and the numerous benign scatterers that produce UT echoes similar to those received from defects.
The above is accomplished in accordance with the hardware scanning control, signal processing, and recording system of the present invention. Briefly, in accordance with the present invention, there is provided a nondestructive evaluation (NDE) system which has considerable accuracy and flexibility. With it, inspection personnel are easily able to perform the above three listed functions. The system uses programmable digital LSI circuitry.